On-board telemetry, tracking and command (TTC) transponders are used for communication for monitoring the condition of a satellite, communication of orbit parameters, and communication for operation control of a satellite, where achievement of communication quality, confidentiality resistance to jamming, and resistance to interference is a key issue.
In recent years, for suppressing jamming and interference to uplink signals for sending control commands from a ground station to a satellite station, the direct sequence spread spectrum method using a long-period spreading code according to the advanced encryption standard (AES) technique is becoming mainstream as described in Non Patent Literature 1. The code period of the long-period spreading code is about 40 years.
In a case where a short-period spreading code is used, the short-period spreading code, which has a short code pattern, may be identified by a third party. If the spreading code is identified, there may be jamming caused by a third party on the ground transmitting a pseudo command signal using the same code pattern as the identified spreading code toward a satellite station, which may cause loss of synchronization of a demodulator in the satellite station or erroneous lock despite a low transmission power density. Alternatively, a command signal using the same spreading code pattern from another satellite communication system may be received as an interference wave, which may cause similar effects.
In contrast, when a long-period spreading code is used, there is no risk that the code pattern of the long-period spreading code is identified by a third party, and such jamming as above will not be received. In addition, since the code pattern is different from those of existing short-period spreading codes, there will be no influence of interference from another satellite communication system.
When the long-period spreading code is used, however, since the spreading code does not have periodicity within time on the order of several milliseconds to several seconds, to which the synchronization techniques for short-period spreading code cannot be applied, establishment of a technique for synchronization of the spreading code at the satellite station will be a problem.
To address the problem, in a method described in Non Patent Literature 1, a ground station spread spectrum modem (GS-SSM) of a ground station demodulates frame synchronization data (FSD) that are periodically transmitted from a satellite station, thereafter adjusts the transmission timing taking a propagation delay between the satellite station and the ground station into account, and adjusts a phase difference between a pseudorandom noise (PN) code phase of the received uplink signal and a PN code generated by the satellite station to be within a delay of approximately 1000 chips, to establish synchronization of the long-period spreading code. In addition, for correct acquisition of codes by an on-board spread spectrum transponder (SST), the reception period of the FSD needs to be longer than the code acquisition time of the SST. The aforementioned technique allows the SST of the satellite station to achieve synchronization independent of the PN code length of an uplink signal like the PN code acquisition with a short-period spreading code.